Re: [Paddlewise] floating tow rope

That is a good argument in favor of braided polypropilene lines. 
They are light and bulky, float, and are easy on the hands. 

The initial 300+lb shock, should be absorbed by a shock cord, not
the paddler nor the boat.

Actually, the 300lb number sounds low to me. The UIAA tests climbing
ropes by letting a 80kg mass fall with 1 meter of rope. That produces
the 2000kg that the rope is expected to withstand. 
A rope would brake for two reasons. Either is does not take 2000Kg,
or it does not stretch enough, which would create a force way over
the 2000Kg pull.

Let us see (boring discussion ahead).

3 knots = 1.5 meter/second (+or-).
50lb kayak + 180lb paddler = 230lb = 104Kg equivalent 

Energy = 1/2 * m * v^2 = 0.5 * 104 * 1.5^2 = 117 Joules to dissipate
                                             ^^^^^^^^^^^^^^^^^^^^^^^
Energy = Force * distance; therefore; Force = Energy / distance

Hence,

For 25 cm stretch, (if the rope stretched with constant force, which
does not; at some intant, therefore, there are even higher forces)

Force = 117 / .25 = 468 Newtons, equivalent to 103lb of shock.
                                               ^^^^^ not too bad.

For 4 cm stretch (no shock cord)

Force = 117 / .04 = 2925 Newtons, equivalent to 644lb of shock.
                                                ^^^^^ not me!, thanks :-{

For 0 stretch, Force = 117 / 0 = infinity

If my physics are too rusty, please let me know ASAP. ;-)

- Julio

> 
> One thing - a little off topic - that I've been thinking about the past
> couple of days is: how strong should the tow rope be?  New rules
> in Canada require "buoyany heaving lines" at least 15 metres long
> on all kayaks.  I've checked the web site Philip has pointed us to
> and can't find anything on how sustantial the heaving line should
> be.  WW paddlers usually use 450 kg (1000 lb) static load for
> rescue ropes and most tow ropes I've seen are about 225 - 300
> kg (500-650 lb approx).  On the other hand, I don't think I want to
> handle more than 100 kg* tension unless I'm on land with a
> solid object to attach to.  Light line is murder on the hands, heavy
> line is easier to pull, but won't break until after it's damaged you.
> I've thought of, say, 250 kg line with a weak point on the tow
> system of 100 kg capacity.  That way under tow it will limit
> its effect on me but the basic line strength is there when I need
> it.
> 
> *100 kg allows for some reserve for impact loading.  When paddling
> a kayak that's still a _lot_ of force.
> 
> Mike
> 
> 
> 
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